1. Field of the Technology
The technology relates to an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses embodying electrophotography are nowadays widely used in many fields, since they realize printing a high-definition image on a recording medium with a simple operation. The image forming apparatus comprises, for example, a photoreceptor, a charging section, an exposure section, a developing section, a transfer section, and a fixing section. The photoreceptor has a photosensitive layer thereon. The charging section charges a surface of the photoreceptor to a predetermined polarity and potential. The exposure section forms an electrostatic latent image on the surface of the photoreceptor in a charged state. The developing section develops the electrostatic latent image on the surface of the photoreceptor using a toner to form a toner image. The transfer section transfers the toner image on the surface of the photoreceptor onto a recording medium. The fixing section fixes the toner image onto the recording medium. Through these processes using the respective sections, an image corresponding to image information is formed on the recording medium.
Here, in the developing section, use is made of a developing device including a developing roller for supplying a toner onto the electrostatic latent image on the surface of the photoreceptor to form the toner image, a developer tank for storing a two-component developer containing the toner therein and supplying the two-component developer onto the developing roller, and a toner concentration sensor for detecting a toner concentration in the developer tank. A toner replenishment into the developer tank is controlled in accordance with a detection result of the toner concentration sensor. The toner concentration sensor normally outputs the detection result as a voltage, which voltage has a tendency to be affected by a detecting sensitivity of the toner concentration sensor itself, use environments (temperature, humidity and accumulated number of times printing) for the two-component developer, and the like. For example, the detecting sensitivity of the toner concentration sensor is changed depending on a temperature, humidity, and the like. In addition, the detecting sensitivity of the toner concentration sensor is also changed depending on speed of printing an image, number of times of printing images, and the like in the image forming apparatus. Moreover, in a color image forming apparatus, a detecting result of the toner concentration sensor is also changed depending on color of toner. Therefore, an appropriate amount of toner may not be replenished into the developer tank, causing a decrease in an image concentration, a faint and patchy image, and the like.
In view of the problem of the related art, there is proposed an image forming apparatus comprising, for example, a photoreceptor, a exposure section, a developing section, a toner concentration sensor, a toner replenishment control section, a control section, and a memory section (refer to Japanese Unexamined Patent Publication JP-A 2006-010749, for example). The toner concentration sensor detects a toner concentration in a two-component developer based on permeability of the two-component developer. The toner replenishment control section replenishes a toner to the developing section. The control section controls the toner replenishment control section in accordance with a detection result by the toner concentration sensor. The memory section stores a correction value of a detecting sensitivity depending on number of times of printing based on a fact that the detecting sensitivity of the toner replenishment sensor is changed depending on number of times of printing. According to the image forming apparatus disclosed in JP-A 2006-010749, a voltage outputted from the toner concentration sensor is corrected according to number of times of printing, and depending on the correction value obtained, a toner is replenished to the developing section by the toner replenishment control section. However, the detecting sensitivity of the toner concentration sensor is more affected by a speed of printing than by number of times of printing. Therefore, even though the detecting sensitivity is corrected only by number of times of printing, an appropriate amount of toner cannot be replenished.
In addition, there is proposed an image forming apparatus comprising a toner concentration sensor for detecting a toner concentration in a developer tank based on a change in permeability of a developer, for initializing a detecting sensitivity of a toner concentration sensor using a specific method (refer to Japanese Unexamined Patent Publication JP-A 2006-0566639, for example). According to the specific method, for a two-component developer in a static state or a flowing state to be filled up into a developer tank, a direct-current voltage for adjusting an output operation point of the toner concentration sensor is set to such a value that the output value of the toner concentration sensor is set to a center of an output fluctuation range of the toner concentration sensor, and thereafter the direct-current voltage is further changed from the above-described value by a predetermined amount, during which the output value of the toner concentration sensor is detected. Accordingly, the detecting sensitivity is initialized. A technique disclosed in JP-A 2000-056639 is designed to focus on a change in detecting sensitivity of the toner concentration sensor depending on a bulk density of the developer when the detecting sensitivity of the toner concentration sensor is initialized, to thereby remove an effect of the bulk density of the developer by adopting the above-described method. However, JP-A 2000-056639 relates to initialization of the detecting sensitivity of the toner concentration sensor, and discloses no technical idea for correcting the detecting sensitivity of the toner concentration sensor to be changed over time. This is also evident from, for example, a description that “in the embodiment, it is possible to set variations in a sensor sensitivity of a toner concentration detecting sensor due to its transvariation, its case and bobbin variation, its assembly variation and the like, in a state in which the toner concentration sensor is arranged in a developing container” in lines 1 to 4 in paragraph [0054] of JP-A 2000-056639. That is, JP-A 2000-056639 does not disclose in any way a technical idea in which the detecting sensitivity of the toner concentration detecting sensor is corrected by a change in speed of printing.